In recent years, optical communication network enabling large capacity data communication at high speed is expanding. The optical communication network is assumed to be mounted on a commercial-off-the-shelf device in the future. An electrical input/output optical data transmission cable (optical cable) capable of being used no different from the present electrical cable is desired for the application of large capacity data transfer at higher speed, noise countermeasures, and data transmission between substrates in the device. In view of flexibility, a film light guide is desirably used for the optical cable.
The light guide is formed by a core having a large index of refraction and a clad having a small index of refraction arranged contacting the periphery of the core, and propagates the optical signal entered to the core while repeating total-reflection at the boundary of the core and the clad. The film light guide has flexibility since the core and the clad are made of a flexible polymer material.
In recent years, in particular, realizing the flexible light guide to be mounted on a bendable display and a smaller and thinner commercial-off-the-shelf device with a light guide is being desired. For instance, patent document 1 describes a light guide having a tapered surface with a tapering angle of 45° at the end.
FIGS. 29(a) and 29(b) show a configuration of a conventional light guide, where FIG. 29(a) is a perspective view, and FIG. 29(b) is a cross-sectional view. In FIGS. 29(a) and 29(b), the width direction of the light guide 4′ is the X-axis direction, the height direction is the Y-axis direction, and the longitudinal direction (optical axis direction) is the Z-axis direction.
As shown in FIG. 29(a), the optical path conversion mirror optical path conversion mirror 4′A of the light guide 4′ is not perpendicular to the optical axis (Z-axis), and is a tapered surface cut at the tapering angle of 45°. The optical path conversion mirror optical path conversion mirror 4′A is perpendicular with respect to the YZ plane. The angle (tapering angle; 45°) formed by the optical path conversion mirror optical path conversion mirror 4′A and a bottom surface 4′B of the light guide is uniform in the X-axis direction.
Thus, the signal light emitted from a light emitting portion 101 is reflected at the tapered surface 4′A so that the advancing direction is changed, and transmitted through the light guide 100.
As another configuration of the conventional light guide, patent document 2 describes a configuration in which the shape of the optical path conversion mirror optical path conversion mirror of the light guide is a lens shape. According to such configuration, the signal light emitted from the light emitting portion is reflected at the end face of the lens shape so that the advancing direction is changed to be parallel to the longitudinal direction of the light guide, and transmitted through the light guide.
Lights having various intensity distributions are propagated since the size of the core is large. The conventional light guide has an inclination angle of the optical path conversion mirror surface set so that the optical path of the signal light is converted to be parallel to the light transmitting direction, and the signal light propagating through the light guide is propagated while being collected or diffused.
Problems do not arise in a rigid light guide or a multimode fiber in which the core is circular, but if bend and twist are applied at the light collecting area in a thinner light guide giving great importance to flexibility, the loss of the signal light at the relevant area becomes large compared to when the signal light in the core is uniformed. In the flexible light guide where bend and twist freely occur, the location of the bend and twist is difficult to specify, and a stable signal quality is difficult to ensure in the prior art.    Patent document 1: Japanese Laid-Open Patent Publication “Japanese Laid-Open Patent Publication No. 2000-117465 (date of publication: Apr. 25, 2000).    Patent document 2: Japanese Laid-Open Patent Publication “Japanese Laid-Open Patent Publication No. 2004-258076 (date of publication: Sep. 16, 2004).